We trust science because its claims are based on experience. But experience itself is a subjective reality that seems to elude the objectivity of scientific understanding. We know that our experiences—of seeing red, feeling pain, falling in love and so forth—depend on physical systems like the brain that science can, in principle, exhaustively explain. But it’s hard to make sense of the idea that experiences themselves could be physical. No doubt experiences correlate with objective physical facts, but, as subjective phenomena, how could they be such facts? When I feel intense pain, scientists may be able to observe brain-events that cause my pain, but they cannot observe the very pain that I feel. What science can observe is public and objective; what I feel is private and subjective.

Perhaps, though, it’s hard to understand how experiences could be physical only because we lack the relevant concepts. After all, before scientists developed the concepts of biochemistry, many found it impossible to imagine how organisms could be entirely physical. Now the idea is far more plausible. What’s to say that future scientific developments won’t allow us to understand how experiences can be entirely physical?

Nevertheless, some of the most interesting recent philosophical discussions have centered around two thought experiments suggesting that experiences cannot be physical. These thought-experiments have by no means settled the issue, but they have convinced quite a few philosophers and have posed a serious challenge to the claim that experience is entirely physical.